Long-term cellular dysfunction after focal cerebral ischemia: in vitro analyses

Oxytocin Exhibits Neuroprotective Effects on Hippocampal Cultures under Severe Oxygen-Glucose Deprivation Conditions

Perinatal asphyxia (PA) and hypoxic-ischemic encephalopathy can result in severe, long-lasting neurological deficits. In vitro models, such as oxygen-glucose deprivation (OGD), are used experimentally to investigate neuronal response to metabolic stress. However, multiple variables can affect the severity level of OGD/PA and may confound any measured treatment effect. Oxytocin (OXT) has emerged as a potential neuroprotective agent against the deleterious effects of PA. Previous studies have demonstrated OXT's potential to enhance neuronal survival in immature hippocampal cultures exposed to OGD, possibly by modulating gamma-aminobutyric acid-A receptor activity. Moreover, OXT's precise impact on developing hippocampal neurons under different severities of OGD/PA remains uncertain. In this study, we investigated the effects of OXT (0.1 µM and 1 µM) on 7-day-old primary rat hippocampal cultures subjected to 2 h OGD/sham normoxic conditions. Cell culture viability was determined using the resazurin assay. Our results indicate that the efficacy of 1 µM OXT treatment varied according to the severity of the OGD-induced lesion, exhibiting a protective effect (p = 0.022) only when cellular viability dropped below 49.41% in non-treated OGD cultures compared to normoxic ones. Furthermore, administration of 0.1 µM OXT did not yield significant effects, irrespective of lesion severity (p > 0.05). These findings suggest that 1 µM OXT treatment during OGD confers neuroprotection exclusively in severe lesions in hippocampal neurons after 7 days in vitro. Further research is warranted to elucidate the mechanisms involved in OXT-mediated neuroprotection.

Aberrant cortical activity, functional connectivity, and neural assembly architecture after photothrombotic stroke in mice

Despite substantial progress in mapping the trajectory of network plasticity resulting from focal ischemic stroke, the extent and nature of changes in neuronal excitability and activity within the peri-infarct cortex of mice remains poorly defined. Most of the available data have been acquired from anesthetized animals, acute tissue slices, or infer changes in excitability from immunoassays on extracted tissue, and thus may not reflect cortical activity dynamics in the intact cortex of an awake animal. Here, in vivo two-photon calcium imaging in awake, behaving mice was used to longitudinally track cortical activity, network functional connectivity, and neural assembly architecture for 2 months following photothrombotic stroke targeting the forelimb somatosensory cortex. Sensorimotor recovery was tracked over the weeks following stroke, allowing us to relate network changes to behavior. Our data revealed spatially restricted but long-lasting alterations in somatosensory neural network function and connectivity. Specifically, we demonstrate significant and long-lasting disruptions in neural assembly architecture concurrent with a deficit in functional connectivity between individual neurons. Reductions in neuronal spiking in peri-infarct cortex were transient but predictive of impairment in skilled locomotion measured in the tapered beam task. Notably, altered neural networks were highly localized, with assembly architecture and neural connectivity relatively unaltered a short distance from the peri-infarct cortex, even in regions within 'remapped' forelimb functional representations identified using mesoscale imaging with anaesthetized preparations 8 weeks after stroke. Thus, using longitudinal two-photon microscopy in awake animals, these data show a complex spatiotemporal relationship between peri-infarct neuronal network function and behavioral recovery. Moreover, the data highlight an apparent disconnect between dramatic functional remapping identified using strong sensory stimulation in anaesthetized mice compared to more subtle and spatially restricted changes in individual neuron and local network function in awake mice during stroke recovery.

Transient ischemia-reperfusion induces cortical hyperactivity and AMPAR trafficking in the somatosensory cortex

Brain ischemia results from cardiac arrest, stroke or head trauma. The structural basis of rescuing the synaptic impairment and cortical dysfunctions induced in the stage of ischemic-reperfusion can occur if therapeutic interventions are applied in time, but the functional basis for this resilience remains elusive. Here, we explore the changes in cortical activity and a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) GluA1 subunit in spine (sGluA1) after transient ischemia-reperfusion in vivo for 28 days. Using in vivo two-photon microscopy in the mouse somatosensory cortex, we found that the average frequency of Ca²⁺ transients in the spine (there was an unusual synchrony) was higher after 15 min of ischemia-reperfusion. In addition, the transient ischemia-reperfusion caused a reflective enhancement of AMPARs, which eventually restored to normal. The cortical hyperactivity (Ca²⁺ transients) and the increase in AMPARs were successfully blocked by an NMDA receptor antagonist. Thus, the increase of AMPARs, cortical hyperactivity and the unusual synchrony might be the reason for reperfusion injury after short-term transient ischemia.

The Roles of GABA in Ischemia-Reperfusion Injury in the Central Nervous System and Peripheral Organs

Ischemia-reperfusion (I/R) injury is a common pathological process, which may lead to dysfunctions and failures of multiple organs. A flawless medical way of endogenous therapeutic target can illuminate accurate clinical applications. γ-Aminobutyric acid (GABA) has been known as a marker in I/R injury of the central nervous system (mainly in the brain) for a long time, and it may play a vital role in the occurrence of I/R injury. It has been observed that throughout cerebral I/R, levels, syntheses, releases, metabolisms, receptors, and transmissions of GABA undergo complex pathological variations. Scientists have investigated the GABAergic enhancers for attenuating cerebral I/R injury; however, discussions on existing problems and mechanisms of available drugs were seldom carried out so far. Therefore, this review would summarize the process of pathological variations in the GABA system under cerebral I/R injury and will cover corresponding probable issues and mechanisms in using GABA-related drugs to illuminate the concern about clinical illness for accurately preventing cerebral I/R injury. In addition, the study will summarize the increasing GABA signals that can prevent I/R injuries occurring in peripheral organs, and the roles of GABA were also discussed correspondingly.

Dynamic Reorganization of Motor Networks During Recovery from Partial Spinal Cord Injury in Monkeys

After spinal cord injury (SCI), the motor-related cortical areas can be a potential substrate for functional recovery in addition to the spinal cord. However, a dynamic description of how motor cortical circuits reorganize after SCI is lacking. Here, we captured the comprehensive dynamics of motor networks across SCI in a nonhuman primate model. Using electrocorticography over the sensorimotor areas in monkeys, we collected broadband neuronal signals during a reaching-and-grasping task at different stages of recovery of dexterous finger movements after a partial SCI at the cervical levels. We identified two distinct network dynamics: grasping-related intrahemispheric interactions from the contralesional premotor cortex (PM) to the contralesional primary motor cortex (M1) in the high-γ band (>70 Hz), and motor-preparation-related interhemispheric interactions from the contralesional to ipsilesional PM in the α and low-β bands (10–15 Hz). The strengths of these networks correlated to the time course of behavioral recovery. The grasping-related network showed enhanced activation immediately after the injury, but gradually returned to normal while the strength of the motor-preparation-related network gradually increased. Our findings suggest a cortical compensatory mechanism after SCI, where two interdependent motor networks redirect activity from the contralesional hemisphere to the other hemisphere to facilitate functional recovery.

Enhanced phasic GABA inhibition during the repair phase of stroke: a novel therapeutic target

Ischaemic stroke is the leading cause of severe long-term disability yet lacks drug therapies that promote the repair phase of recovery. This repair phase of stroke occurs days to months after stroke onset and involves brain remapping and plasticity within the peri-infarct zone. Elucidating mechanisms that promote this plasticity is critical for the development of new therapeutics with a broad treatment window. Inhibiting tonic (extrasynaptic) GABA signalling during the repair phase was reported to enhance functional recovery in mice suggesting that GABA plays an important function in modulating brain repair. While tonic GABA appears to suppress brain repair after stroke, less is known about the role of phasic (synaptic) GABA during the repair phase. We observed an increase in postsynaptic phasic GABA signalling in mice within the peri-infarct cortex specific to layer 5; we found increased numbers of α1 receptor subunit-containing GABAergic synapses detected using array tomography, and an associated increased efficacy of spontaneous and miniature inhibitory postsynaptic currents in pyramidal neurons. Furthermore, we demonstrate that enhancing phasic GABA signalling using zolpidem, a Food and Drug Administration (FDA)-approved GABA-positive allosteric modulator, during the repair phase improved behavioural recovery. These data identify potentiation of phasic GABA signalling as a novel therapeutic strategy, indicate zolpidem’s potential to improve recovery, and underscore the necessity to distinguish the role of tonic and phasic GABA signalling in stroke recovery.

Time-sensitive reorganization of the somatosensory cortex poststroke depends on interaction between Hebbian and homeoplasticity: a simulation study

Together with Hebbian plasticity, homeoplasticity presumably plays a significant, yet unclear, role in recovery postlesion. Here, we undertake a simulation study addressing the role of homeoplasticity and rehabilitation timing poststroke. We first hypothesize that homeoplasticity is essential for recovery and second that rehabilitation training delivered too early, before homeoplasticity has compensated for activity disturbances postlesion, is less effective for recovery than training delivered after a delay. We developed a neural network model of the sensory cortex driven by muscle spindle inputs arising from a six-muscle arm. All synapses underwent Hebbian plasticity, while homeoplasticity adjusted cell excitability to maintain a desired firing distribution. After initial training, the network was lesioned, leading to areas of hyper- and hypoactivity due to the loss of lateral synaptic connections. The network was then retrained through rehabilitative arm movements. We found that network recovery was unsuccessful in the absence of homeoplasticity, as measured by reestablishment of lesion-affected inputs. We also found that a delay preceding rehabilitation led to faster network recovery during the rehabilitation training than no delay. Our simulation results thus suggest that homeoplastic restoration of prelesion activity patterns is essential to functional network recovery via Hebbian plasticity.

Adult cortical plasticity following injury: Recapitulation of critical period mechanisms?

A primary goal of research on developmental critical periods (CPs) is the recapitulation of a juvenile-like state of malleability in the adult brain that might enable recovery from injury. These ambitions are often framed in terms of the simple reinstatement of enhanced plasticity in the growth-restricted milieu of an injured adult brain. Here, we provide an analysis of the similarities and differences between deprivation-induced and injury-induced cortical plasticity, to provide for a nuanced comparison of these remarkably similar processes. As a first step, we review the factors that drive ocular dominance plasticity in the primary visual cortex of the uninjured brain during the CP and in adults, to highlight processes that might confer adaptive advantage. In addition, we directly compare deprivation-induced cortical plasticity during the CP and plasticity following acute injury or ischemia in mature brain. We find that these two processes display a biphasic response profile following deprivation or injury: an initial decrease in GABAergic inhibition and synapse loss transitions into a period of neurite expansion and synaptic gain. This biphasic response profile emphasizes the transition from a period of cortical healing to one of reconnection and recovery of function. Yet while injury-induced plasticity in adult shares several salient characteristics with deprivation-induced plasticity during the CP, the degree to which the adult injured brain is able to functionally rewire, and the time required to do so, present major limitations for recovery. Attempts to recapitulate a measure of CP plasticity in an adult injury context will need to carefully dissect the circuit alterations and plasticity mechanisms involved while measuring functional behavioral output to assess their ultimate success.

Discrepancy between voluntary movement and motor-evoked potentials in evaluation of motor function during clipping of anterior circulation aneurysms

BACKGROUND

Various modalities have been used to confirm the blood flow through parent arteries or surrounding perforating arteries during surgical aneurysm clipping, including motor-evoked potentials (MEPs), Doppler ultrasound, and indocyanine green videoangiography. Nonetheless, contralateral hemiparesis due to arterial blood flow insufficiency may arise because of false-positive or false-negative errors. By performing controlled intraoperative awakening during aneurysm clipping, we compared patients' voluntary movements with simultaneous MEP.

METHODS

Four patients with anterior choroidal artery aneurysms and one patient with a dorsal internal carotid artery aneurysm were included in this study. MEP and intraoperative voluntary movements under awake craniotomy were assessed simultaneously during and after the clipping procedure.

RESULTS

Aneurysms were safely and successfully clipped in all patients, with no evidence of postoperative neurological deficits. Voluntary movements and MEP findings did not differ from the control state in three patients. In the other two patients, we observed a discrepancy between MEP amplitudes and voluntary movements. In one patient, deterioration and subsequent improvement in voluntary movements were preceded by MEP amplitude reduction during clipping. In the other patient, MEP amplitude did not change although voluntary movement deteriorated during temporary occlusion of the internal carotid artery.

CONCLUSIONS

Intraoperative neurological assessment during aneurysmal clipping under awake craniotomy is feasible and safe, and should be valuable for the assessment of ischemia, especially in the anterior choroidal artery. From a neurophysiologic viewpoint, MEP may be insufficiently sensitive for evaluating voluntary movement under ischemia.

Influence of intra- and extracellular acidification on free radical formation and mitochondria membrane potential in rat brain synaptosomes

Brain ischemia is accompanied by lowering of intra- and extracellular pH. Stroke often leads to irreversible damage of synaptic transmission by unknown mechanism. We investigated an influence of lowering of pH(i) and pH(o) on free radical formation in synaptosomes. Three models of acidosis were used: (1) pH(o) 6.0 corresponding to pH(i) decrease down to 6.04; (2) pH(o) 7.0 corresponding to the lowering of pH(i) down to 6.92: (3) 1 mM amiloride corresponding to pH(i) decrease down to 6.65. We have shown that both types of extracellular acidification, but not intracellular acidification, increase 2',7'-dichlorodihydrofluorescein diacetate fluorescence that reflects free radical formation. These three treatments induce the rise of the dihydroethidium fluorescence that reports synthesis of superoxide anion. However, the impact of amiloride on superoxide anion synthesis was less than that induced by moderate extracellular acidification. Superoxide anion synthesis at pH(o) 7.0 was almost completely eliminated by mitochondrial uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone. Furthermore, using fluorescent dyes JC-1 and rhodamine-123, we confirmed that pH(o) lowering, but not intracellular acidification, led to depolarization of intrasynaptosomal mitochondria. We have shown that pH(o) but not pH(i) lowering led to oxidative stress in neuronal presynaptic endings that might underlie the long-term irreversible changing in synaptic transmission.

Ischemic cerebral damage: an appraisal of synaptic failure

In the human brain, ≈30% of the energy is spent on synaptic transmission. Disappearance of synaptic activity is the earliest consequence of cerebral ischemia. The changes of synaptic function are generally assumed to be reversible and persistent damage is associated with membrane failure and neuronal death. However, there is overwhelming experimental evidence of isolated, but persistent, synaptic failure resulting from mild or moderate cerebral ischemia. Early failure results from presynaptic damage with impaired transmitter release. Proposed mechanisms include dysfunction of adenosine triphosphate-dependent calcium channels and a disturbed docking of glutamate-containing vesicles resulting from impaired phosphorylation. We review energy distribution among neuronal functions, focusing on energy usage of synaptic transmission. We summarize the effect of ischemia on neurotransmission and the evidence of long-lasting synaptic failure as a cause of persistent symptoms in patients with cerebral ischemia. Finally, we discuss the implications of synaptic failure in the diagnosis of cerebral ischemia, including the limited sensitivity of diffusion-weighted MRI in those cases in which damage is presumably limited to the synapses.

Mechanism of functional recovery after repetitive transcranial magnetic stimulation (rTMS) in the subacute cerebral ischemic rat model: neural plasticity or anti-apoptosis?

Repetitive transcranial magnetic stimulation (rTMS) has been studied increasingly in recent years to determine whether it has a therapeutic benefit on recovery after stroke. However, the underlying mechanisms of rTMS in stroke recovery remain unclear. Here, we evaluated the effect of rTMS on functional recovery and its underlying mechanism by assessing proteins associated with neural plasticity and anti-apoptosis in the peri-lesional area using a subacute cerebral ischemic rat model. Twenty cerebral ischemic rats were randomly assigned to the rTMS or the sham group at post-op day 4. A total of 3,500 impulses with 10 Hz frequency were applied to ipsilesional cortex over a 2-week period. Functional outcome was measured before (post-op day 4) and after rTMS (post-op day 18). The rTMS group showed more functional improvement on the beam balance test and had stronger Bcl-2 and weaker Bax expression on immunohistochemistry compared with the sham group. The expression of NMDA and MAP-2 showed no significant difference between the two groups. These results suggest that rTMS in subacute cerebral ischemia has a therapeutic effect on functional recovery and is associated with an anti-apoptotic mechanism in the peri-ischemic area rather than with neural plasticity.

Motor-evoked potential confirmation of functional improvement by transplanted bone marrow mesenchymal stem cell in the ischemic rat brain

This study investigated the effect of bone marrow mesenchymal stem cells (BMSCs) on the motor pathway in the transient ischemic rat brain that were transplanted through the carotid artery, measuring motor-evoked potential (MEP) in the four limbs muscle and the atlantooccipital membrane, which was elicited after monopolar and bipolar transcortical stimulation. After monopolar stimulation, the latency of MEP was significantly prolonged, and the amplitude was less reduced in the BMSC group in comparison with the control group (P < .05). MEPs induced by bipolar stimulation in the left forelimb could be measured in 40% of the BMSC group and the I wave that was not detected in the control group was also detected in 40% of the BMSC group. Our preliminary results imply that BMSCs transplanted to the ischemic rat brain mediate effects on the functional recovery of the cerebral motor cortex and the motor pathway.

Functional consequences of the disturbances in the GABA-mediated inhibition induced by injuries in the cerebral cortex

Cortical injuries are often reported to induce a suppression of the intracortical GABAergic inhibition in the surviving, neighbouring neuronal networks. Since GABAergic transmission provides the main source of inhibition in the mammalian brain, this condition may lead to hyperexcitability and epileptiform activity of cortical networks. However, inhibition plays also a crucial role in limiting the plastic properties of neuronal circuits, and as a consequence, interventions aiming to reestablish a normal level of inhibition might constrain the plastic capacity of the cortical tissue. A promising strategy to minimize the deleterious consequences of a modified inhibitory transmission without preventing the potential beneficial effects on cortical plasticity may be to unravel distinct GABAergic signaling pathways separately mediating these positive and negative events. Here, gathering data from several recent studies, we provide new insights to better face with this "double coin" condition in the attempt to optimize the functional recovery of patients.

Changes in intracortical excitability after transient ischemic attack are associated with ABCD² score

BACKGROUND AND PURPOSE

A transient ischemic attack (TIA) is a brief ischemic episode characterized by rapid clinical resolution and not associated with permanent cerebral infarction. Whether changes in intracortical excitability persist and are related to clinical predictors of stroke risk after TIA remains unknown.

METHODS

Participants were individuals with clinically resolved motor TIA with no structural lesions and healthy age-matched control participants. Single and paired-pulse transcranial magnetic stimulation was used to measure intracortical excitability. Recruitment curves for percent inhibition and facilitation were used to derive excitability thresholds. Correlations between threshold asymmetries and ABCD(2) score were performed.

RESULTS

Results showed a significant 3-way interaction with reduced inhibition and enhanced facilitation in the affected compared with unaffected hemisphere after TIA. No significant differences were present in healthy participants. Asymmetries in intracortical inhibition and facilitation were significantly correlated with ABCD(2) score.

CONCLUSIONS

The present study is the first, to our knowledge, to demonstrate altered intracortical inhibition and facilitation in the affected hemisphere after TIA. These changes occurred on average 2 weeks after clinical signs of TIA resolved and in the absence of structural lesions and were not present in healthy age-matched control participants. Furthermore, this study is the first, to our knowledge, to report that changes in intracortical excitability after TIA are associated with ABCD(2) score.

Long-lasting inhibition of presynaptic metabolism and neurotransmitter release by protein S-nitrosylation

Nitric oxide (NO) and related reactive nitrogen species (RNS) play a major role in the pathophysiology of stroke and other neurodegenerative diseases. One of the poorly understood consequences of stroke is a long-lasting inhibition of synaptic transmission. In this study, we tested the hypothesis that RNS can produce long-term inhibition of neurotransmitter release via S-nitrosylation of proteins in presynaptic nerve endings. We examined the effects of exogenous sources of RNS on the vesicular and nonvesicular L-[(3)H]glutamate release from rat brain synaptosomes. NO/RNS donors, such as spermine NONOate, MAHMA NONOate, S-nitroso-L-cysteine, and SIN-1, inhibited only the vesicular component of glutamate release with an order of potency that closely matched levels of protein S-nitrosylation. Inhibition of glutamate release persisted for >1h after RNS donor decomposition and washout and strongly correlated with decreases in the intrasynaptosomal ATP levels. Post-NO treatment of synaptosomes with thiol-reducing reagents decreased the total content of S-nitrosylated proteins but had little effect on glutamate release and ATP levels. In contrast, post-NO application of the end-product of glycolysis, pyruvate, partially rescued neurotransmitter release and ATP production. These data suggest that RNS suppress presynaptic metabolism and neurotransmitter release via poorly reversible modifications of glycolytic and mitochondrial enzymes, one of which was identified as glyceraldehyde-3-phosphate dehydrogenase. A similar mechanism may contribute to the long-term suppression of neuronal communication during nitrosative stress in vivo.

Plasticity during stroke recovery: from synapse to behaviour

Reductions in blood flow to the brain of sufficient duration and extent lead to stroke, which results in damage to neuronal networks and the impairment of sensation, movement or cognition. Evidence from animal models suggests that a time-limited window of neuroplasticity opens following a stroke, during which the greatest gains in recovery occur. Plasticity mechanisms include activity-dependent rewiring and synapse strengthening. The challenge for improving stroke recovery is to understand how to optimally engage and modify surviving neuronal networks, to provide new response strategies that compensate for tissue lost to injury.

Remapping the somatosensory cortex after stroke: insight from imaging the synapse to network

Together, thousands of neurons with similar function make up topographically oriented sensory cortex maps that represent contralateral body parts. Although this is an accepted model for the adult cortex, whether these same rules hold after stroke-induced damage is unclear. After stroke, sensory representations damaged by stroke remap onto nearby surviving neurons. Here, we review the process of sensory remapping after stroke at multiple levels ranging from the initial damage to synapses, to their rewiring and function in intact sensory circuits. We introduce a new approach using in vivo 2-photon calcium imaging to determine how the response properties of individual somatosensory cortex neurons are altered during remapping. One month after forelimb-area stroke, normally highly limb-selective neurons in surviving peri-infarct areas exhibit remarkable flexibility and begin to process sensory stimuli from multiple limbs as remapping proceeds. Two months after stroke, neurons within remapped regions develop a stronger response preference. Thus, remapping is initiated by surviving neurons adopting new roles in addition to their usual function. Later in recovery, these remapped forelimb-responsive neurons become more selective, but their new topographical representation may encroach on map territories of neurons that process sensory stimuli from other body parts. Neurons responding to multiple limbs may reflect a transitory phase in the progression from their involvement in one sensorimotor function to a new function that replaces processing lost due to stroke.

In vivo calcium imaging reveals functional rewiring of single somatosensory neurons after stroke

Functional mapping and microstimulation studies suggest that recovery after stroke damage can be attributed to surviving brain regions taking on the functional roles of lost tissues. Although this model is well supported by data, it is not clear how activity in single neurons is altered in relation to cortical functional maps. It is conceivable that individual surviving neurons could adopt new roles at the expense of their usual function. Alternatively, neurons that contribute to recovery may take on multiple functions and exhibit a wider repertoire of neuronal processing. In vivo two-photon calcium imaging was used in adult mice within reorganized forelimb and hindlimb somatosensory functional maps to determine how the response properties of individual neurons and glia were altered during recovery from ischemic damage over a period of 2-8 weeks. Single-cell calcium imaging revealed that the limb selectivity of individual neurons was altered during recovery from ischemia, such that neurons normally selective for a single contralateral limb processed information from multiple limbs. Altered limb selectivity was most prominent in border regions between stroke-altered forelimb and hindlimb macroscopic map representations, and peaked 1 month after the targeted insult. Two months after stroke, individual neurons near the center of reorganized functional areas became more selective for a preferred limb. These previously unreported forms of plasticity indicate that in adult animals, seemingly hardwired cortical neurons first adopt wider functional roles as they develop strategies to compensate for loss of specific sensory modalities after forms of brain damage such as stroke.

Post-hypoxic changes in rat cortical neuron GABA A receptor function require L-type voltage-gated calcium channel activation

Hypoxia modifies GABA(A) receptor (GABA(A)R) function and can cause seizures, encephalopathy or myoclonus. To characterize the effects of hypoxia on neuronal GABA(A)Rs, we subjected rat cortical neurons to 1% O2 for 2, 4 or 8h, followed by recovery times of 0-96h, and used whole-cell and perforated patch-clamp recording to assess GABA(A)R currents and pharmacology. Hypoxic exposure for 4h caused downregulation of maximal GABA current immediately following hypoxia and after 48h recovery without changing the EC50 for GABA. Two- and eight-hour hypoxic exposures had inconsistent effects on GABA(A)R currents. Maximal diazepam potentiation was increased immediately following 4h hypoxia, while potentiation by zolpidem was increased after 48h recovery. Pentobarbital enhancement and zinc inhibition of GABA currents were unchanged. Hypoxia also caused a depolarizing shift in the reversal potential of GABA-induced Cl(-) currents after 24h recovery. The L-type voltage-gated calcium channel (L-VGCC) blocker, nitrendipine, during hypoxia or control treatment prevented the reduction in GABA(A)R currents, and increased control currents over baseline. Nitrendipine also prevented the increase in zolpidem potentiation 48h after hypoxia, and blocked the depolarizing shift in Cl(-) reversal potential 24h after hypoxia. The effects of hypoxia on maximal GABA(A)R currents, zolpidem pharmacology and Cl(-) reversal potential thus require depolarization-induced calcium entry via L-VGCCs, and constitutive L-VGCC activity appears to reduce maximal GABA(A)R currents in control neurons via a calcium-dependent process. Calcium-dependent modulation of GABA(A)R currents via L-VGCCs may be a fundamental regulatory mechanism for GABA receptor function.

CNS-active drugs in aging population at high risk of cerebrovascular events: evidence from preclinical and clinical studies

The recovery process following cerebral insults such as stroke is affected by aging and pharmacotherapy. The use of medication including CNS-active drugs has increased in the elderly during recent years. However, surprisingly little is known about how safe they are with respect to severity of sensorimotor and cognitive impairments or recovery of function following possible cerebrovascular accidents. This review examines the experimental and clinical literature, primarily from 1995 onwards, concerning medication in relation to cerebrovascular events and functional recovery. Special attention is directed to polypharmacy and to new CNS-active drugs, which the elderly are already taking or are prescribed to treat emerging, stroke-induced psychiatric symptoms. The neurobiological mechanisms affected by these drugs are discussed.

Erratum to "Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintenance of epilepsy." [Pharmacol. Ther. 105(3) (2005) 229-266]

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury [central nervous system (CNS) insult]. (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels ([Ca(2+)](i)) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but the share a common molecular mechanism for producing brain damage--an increase in extracellular glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Run down of GABAergic depolarization during metabolic inhibition of rat hippocampal CA1 neurons

We investigated the effects of metabolic inhibition on both the shift in the equilibrium potential for Cl(-) (E(Cl)) and the run down of GABA(A) receptor responses, using nystatin- and gramicidin-perforated patch-clamp recordings from rat hippocampal CA1 neurons. Metabolic inhibition with NaCN decreased outward GABAergic currents while increasing inward GABAergic currents. E(Cl) showed a positive shift almost immediately after metabolic poisoning. This shift always occurred prior to GABA receptor run down, which was observed as decreases in whole cell conductance during application of a GABA(A) receptor agonist. The results indicate that GABAergic responses tend to become depolarizing during metabolic inhibition and the run down of the GABAergic response may therefore be neuroprotective against excitotoxicity. Furthermore the results illustrate the importance of considering both changes in receptor function and current driving force, and their temporal relationship, in order to understand the physiological response of the GABAergic system during metabolic stress.

Ongoing epileptiform activity in the post-ischemic hippocampus is associated with a permanent shift of the excitatory-inhibitory synaptic balance in CA3 pyramidal neurons

Ischemic strokes are often associated with late-onset epilepsy, but the underlying mechanisms are poorly understood. In the hippocampus, which is one of the regions most sensitive to ischemic challenge, global ischemia induces a complete loss of CA1 pyramidal neurons, whereas the resistant CA3 pyramidal neurons display a long-term hyperexcitability several months after the insult. The mechanisms of this long-term hyperexcitability remain unknown despite its clinical implication. Using chronic in vivo EEG recordings and in vitro field recordings in slices, we now report spontaneous interictal epileptiform discharges in the CA3 area of the hippocampus from post-ischemic rats several months after the insult. Whole-cell recordings from CA3 pyramidal neurons, revealed a permanent reduction in the frequency of spontaneous and miniature GABAergic IPSCs and a parallel increase in the frequency of spontaneous and miniature glutamatergic postsynaptic currents. Global ischemia also induced a dramatic loss of GABAergic interneurons and terminals together with an increase in glutamatergic terminals in the CA3 area of the hippocampus. Altogether, our results show a morpho-functional reorganization in the CA3 network several months after global ischemia, resulting in a net shift in the excitatory-inhibitory balance toward excitation that may constitute a substrate for the generation of epileptiform discharges in the post-ischemic hippocampus.

Ischemic preconditioning ameliorates excitotoxicity by shifting glutamate/gamma-aminobutyric acid release and biosynthesis

Excitotoxicity is recognized to play a major role in cerebral ischemia-induced cell death. The main goal of the present study was to define whether our model of ischemic preconditioning (IPC) promotes a shift from excitatory to inhibitory neurotransmission during the test ischemia to diminish metabolic demand during the reperfusion phase. We also determined whether gamma-aminobutyric acid (GABA) played a role in IPC-induced neuroprotection. Ten minutes of cerebral ischemia was produced by tightening the carotid ligatures bilaterally following hypotension. Samples of microdialysis perfusate, representing extracellular fluid, were analyzed for amino acid content by HPLC. IPC promoted a robust release of GABA after lethal ischemia compared with control rats. We also observed that the activity of glutamate decarboxylase (the predominant pathway of GABA synthesis in the brain) was higher in the IPC group compared with control and ischemic groups. Because GABAA receptor up-regulation has been shown to occur following IPC, and GABAA receptor activation has been implicated in neuroprotection against ischemic insults, we tested the hypothesis that GABAA or GABAB receptor activation was neuroprotective during ischemia or early reperfusion by using an in vitro model (organotypic hippocampal slice culture). Administration of the GABAB agonist baclofen during test ischemia and for 1 hr of reperfusion provided significant neuroprotection. We concluded that increased GABA release in preconditioned animals after ischemia might be one of the factors responsible for IPC neuroprotection. Specific activation of GABAB receptor contributes significantly to neuroprotection against ischemia in organotypic hippocampal slices.

Stable expression of the vesicular GABA transporter following photothrombotic infarct in rat brain

Before exocytotic release of the inhibitory neurotransmitter GABA, this amino acid has to be stored in synaptic vesicles. Accumulation of GABA in vesicles is achieved by a specific membrane-integrated transporter termed vesicular GABA transporter. This vesicular protein is mainly located at presynaptic terminals of GABAergic interneurons. In the present study we investigated the effects of focal ischemia on the expression of the vesicular GABA transporter. Vesicular GABA transporter mRNA and protein expression was examined after photothrombosis in different cortical and hippocampal brain regions of Wistar rats. In situ hybridization and quantitative real-time RT-PCR were performed to analyze vesicular GABA transporter mRNA. Both vesicular GABA transporter mRNA-stained perikarya and mRNA expression levels remained unaffected. Vesicular GABA transporter protein-containing synaptic terminals and somata were visualized by immunohistochemistry. The pattern of vesicular GABA transporter immunoreactivity as well as the protein expression level revealed by semiquantitative image analysis and by Western blot remained stable after stroke. The steady expression of vesicular GABA transporter mRNA and protein after photothrombosis indicates that the exocytotic release mechanism of GABA is not affected by ischemia.

Pathophysiology of Stroke Rehabilitation: Temporal Aspects of Neurofunctional Recovery

Stroke almost always causes an impairment of motor activity and function. Clinical recovery, though usually incomplete, is often highly dynamic and reflects the ability of the neuronal network to adapt. Mechanisms that underlie neuro-functional plasticity are now beginning to be understood. Albeit the enormous efforts undertaken to support the natural course of re-convalescence through rehabilitation, little has been done to relate possible effects of these therapeutic approaches to mechanisms of adaptive pathophysiology. The review presented here focuses on these mechanisms during the course of recovery post stroke. Next to an unmasking of latent network representations, other adaptive processes, such as excitatory metabolic stress, an imbalance in activating and inhibiting transmission, leading to salient hyperexcitability or mechanisms that consolidate novel connections prime the system's plastic capabilities. These pathophysiological processes potentially interact with rehabilitative interventions. They therefore form the foundation of positive, but possibly also negative recuperation under therapy.

Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintainance of epilepsy

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury (central nervous system [CNS] insult), (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels [Ca(2+)](i) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but they share a common molecular mechanism for producing brain damage-an increase in extracellular glutamate concentration that causes increased intracellular neuronal calcium, leading to neuronal injury and/or death. Neurons that survive the injury induced by glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Insulin exerts neuroprotection by counteracting the decrease in cell-surface GABAA receptors following oxygen-glucose deprivation in cultured cortical neurons

A loss of balance between excitatory and inhibitory signaling leads to excitoxicity, and contributes to ischemic cell death. Reduced synaptic inhibition as a result of dysfunction of the ionotropic GABAA receptor has been suggested as one of the major causes for this imbalance, although the underlying mechanisms remain poorly understood. In the present study, we investigated whether oxygen-glucose deprivation (OGD), an ischemia-like challenge, alters cell-surface expression of GABAA receptors in cultured hippocampal neurons, and thereby leads to excitotoxic cell death. Using cell culture ELISA as a cell surface receptor assay, we found that OGD produced a marked decrease in cell surface GABAA receptors, without altering the total amount of receptors. Furthermore, the reduction could be prevented by inhibition of receptor endocytosis with hypertonic sucrose treatment. Notably, insulin significantly limited OGD-induced changes in cell-surface GABAA receptors. In parallel, insulin protected cultured neurons against both glutamate toxicity and OGD, as assayed by mitochondrial reduction of Alamar Blue. Importantly, insulin-mediated neuroprotection was eliminated when bicuculline, a GABAA receptor antagonist, was co-applied with insulin during OGD. Together, our results strongly suggest that ischemia-like insults decrease cell surface GABAA receptors in neurons via accelerated internalization, and that insulin provides neuroprotection by counteracting this reduction.

Neuro-overprotection? A functional evaluation of clomethiazole-induced neuroprotection following hypoxic-ischemic injury

Hypoxic-ischemic (H-I) injury produces extensive damage to the hippocampus of young rats. We have recently shown that administration of 125 mg kg-1 clomethiazole (CMZ), a GABA(A)-agonist, provides complete histological protection against H-I injury if administered 3 h post-H-I (Brain Res 1035 (2005) 194). However, whether that histological protection translates into lasting functional preservation is unclear. To determine whether hippocampal-based circuits remain functionally intact in CMZ-protected H-I rats, we administered 125 mg kg-1 (high dose [CMZ-HD]) or 65 mg kg-1 (low dose [CMZ-LD]) CMZ, 3 h post-H-I, and examined numerous kindling parameters in the dorsal hippocampus 60 days following H-I. Kindling parameters included afterdischarge (AD) thresholds (ADTs), AD durations and kindling rates. Additional groups assessed included vehicle-injected H-I (VIH), hypoxic, ligated and naive rats. VIH, CMZ-HD, CMZ-LD and hypoxic rats all exhibited significantly faster kindling rates than naive rats. Thus, a previous traumatic event, even hypoxia alone, facilitated subsequent seizure propagation. Still, a significantly slower kindling rate was evident in CMZ-HD rats than in hypoxic, VIH or CMZ-LD rats. Moreover, while longer pre-kindling AD durations were observed in the damaged hippocampus of VIH compared with naive rats, this was not true for either CMZ-treated groups, hypoxic or ligated rats. Collectively, these findings suggest CMZ can suppress the epileptogenic effects of H-I. Surprisingly, however, both groups of CMZ-treated rats exhibited a four to nine times greater ADT than any other group and this effect was most profound in the CMZ-protected hippocampus. Thus, CMZ administration protected local neurons against terminal insult and left network excitability relatively normal with respect to seizure offset mechanisms but also caused profound elevation of local ADTs, which suggests a local hypoexcitability/increased inhibition. Finally, this study demonstrates, for the first time, that the kindling model can serve as a sensitive measure of function-related neuroprotective efficacy in animal models of ischemia.

Augmentation of hypoxic respiration after brief hyperoxia in the anesthetized cat: Putative function of GABAA neurotransmission

In this study, we attempted to determine the role of GABA neurotransmission in augmentation of hypoxic respiration by antecedent hyperoxic breathing. The experiments were performed in anesthetized, paralyzed and vagotomized cats divided into control and bicuculline (a GABA(A) receptor blocker)-injected groups. The experimental protocol consisted of exposing the animals to successive hypoxic-hyperoxic-hypoxic conditions. Respiration was assessed using phrenic electroneurograms, from which the peak phrenic height, a surrogate of the tidal volume component, and respiratory rate were obtained, and their product, the respiratory minute output, was calculated. We found that prior hyperoxic ventilation increased the subsequent respiratory response to hypoxia by an average of 23.5%, compared with the preoxygen response. This increase was driven by volume respiration. The biphasic character of the hypoxic respiratory response, consisting of stimulatory and depressant phases, was sustained. Bicuculline abolished the augmentative effect on hypoxic respiration of prior hyperoxia, which suggests that oxygenation induces GABA(A)-mediated hyperexcitability of respiratory neurons, possibly by the liberation of reactive oxygen species. We concluded that GABA neurotransmission is pertinent to the effect of hyperoxia on hypoxic respiratory reactivity.

Ischemia induces short- and long-term remodeling of synaptic activity in the hippocampus

One of the most vulnerable areas to ischemia or hypoglycemia is CA1 hippocampal region due to pyramidal neurons death. Glutamate receptors are involved together with protein-kinase C and nitric oxide synthase. Long-term potentiation (LTP) is generated in anoxic or hypoglycemic conditions via activation of NMDA while inhibition of these receptors attenuates this response. Protein-kinase C and nitric oxide synthase are involved in anoxic LTP mechanism. Postischemic neurons are hyperexcitable in CA3 area while CA1 pyramidal neurons degenerate and disappear. Changes of glutamate receptors triggered by ischemia and hypoglycemia are discussed in this review.

Synchronous neuronal activity is a signal for axonal sprouting after cortical lesions in the adult

The ability of the adult brain to form new connections in areas denervated by a lesion (axonal sprouting) is more widespread than previously thought, but mechanisms remain unknown. We have previously demonstrated an unexpected, robust axonal sprouting of contralateral corticostriatal neurons into the denervated striatum after ischemic cortical lesions. We now take advantage of marked differences in the degree of axonal sprouting from contralateral homotypic cortex after two types of cortical lesions to define the role of neuronal activity in this response. Thermal-ischemic lesions (TCL) of sensorimotor cortex, which induce axonal sprouting, produced two sequential patterns of low-frequency, synchronized neuronal activity that are not seen after similarly sized aspiration lesions, which do not induce axonal sprouting. An early rhythm of synchronous neuronal activity occurred in perilesion cortex on day 1 after lesion, with a frequency range of 0.2-2 Hz. A later pattern of activity occurred on days 2 and 3 after lesion, with a frequency range of 0.1-0.4 Hz. This second rhythm synchronized neuronal activity across widespread areas, including the cortical areas that contain the cell bodies of the sprouting axons. TTX was used to block this patterned neuronal activity and determine whether axonal sprouting was prevented. Chronic TTX infusion into the lesion site blocked the synchronous neuronal activity after TCL as well as axonal sprouting. Thus, both after different types of lesions and in the blockade experiments axonal sprouting was strongly correlated with synchronous neuronal activity, suggesting a role for this activity in anatomical reorganization after brain lesion in the adult.

Persistent defect in transmitter release and synapsin phosphorylation in cerebral cortex after transient moderate ischemic injury

BACKGROUND AND PURPOSE

Synaptic transmission is highly vulnerable to metabolic perturbations. However, the long-term consequences of transient metabolic perturbations on synapses are not clear. We studied the long-lasting changes in synaptic transmission and phosphorylation of presynaptic proteins in penumbral cortical neurons after transient moderate ischemia.

METHODS

Rats were subjected to 1 hour of middle cerebral artery occlusion. After reperfusion, electric activity of neurons in the peri-infarct region was recorded intracellularly and extracellularly in situ. Phosphorylation of synapsin-I and tyrosine residues was studied by immunohistochemistry.

RESULTS

Neurons in the penumbra displayed no postsynaptic potentials 1 to 3 hours after recirculation. However, these cells were able to generate action potentials and were responsive to glutamate, suggesting that postsynaptic excitability was preserved but the synaptic transmission was blocked because of a presynaptic defect. The synaptic transmission was still depressed 24 hours after recirculation in neurons in the peri-infarct area that survived ischemia. The amount of immunoreactive synapsin-I, synaptophysin, and synaptotagmin was not appreciably changed for 72 hours after reperfusion. However, phosphorylation of synapsin-l was significantly decreased, whereas phosphotyrosine immunoreactivity was increased, suggesting a selective defect in synapsin-I phosphorylation.

CONCLUSIONS

These data demonstrate that synaptic transmission may be permanently impaired after transient moderate brain injury. Since postsynaptic excitability is preserved, the transmission failure is likely to be caused by presynaptic mechanisms, one of which may be impaired phosphorylation of presynaptic proteins.

Neuronal plasticity and dendritic spines: effect of environmental enrichment on intact and postischemic rat brain

The authors compared the influence of environmental enrichment on intact and lesioned brain, and tested the hypothesis that postischemic exposure to an enriched environment can alter dendritic spine density in pyramidal neurons contralateral to a cortical infarct. The middle cerebral artery was occluded distal to the striatal branches in spontaneously hypertensive rats postoperatively housed either in a standard or in an enriched environment. Intact rats were housed in the same environment. Three weeks later the brains were perfused in situ. The dendritic and spine morphology was studied with three-dimensional confocal laser scanning microscopy after microinjection of Lucifer yellow in pyramidal neurons in layers II/III and V/VI in the somatosensory cortex. In intact rats, the number of dendritic spines was significantly higher in the enriched group than in the standard group in all layers ( P < 0.05). Contralateral to the infarct, pyramidal neurons in layers II/III, which have extensive intracortical connections that may play a role in cortical plasticity, had significantly more spines in the enriched group than in the standard group ( P < 0.05). No difference was observed in layers V/VI. They conclude that housing rats in an enriched environment significantly increases spine density in superficial cortical layers in intact and lesioned brain, but in deeper layers of intact brain.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

Altered mechanisms of motor-evoked potential generation after transient focal cerebral ischemia in the rat: implications for transcranial magnetic stimulation

We recently demonstrated that a long-lasting transmission defect in cortical synapses caused motor dysfunction after brief middle cerebral artery (MCA) occlusion in the rat despite rapid recovery of axons. In this experimental study, we have examined the impact of differential recovery of synapses and axons on generation of motor-evoked potentials (MEP) recorded from contralateral paralyzed and ipsilateral unaffected muscles, to gain insight into mechanisms of MEPs recorded from stroke patients by transcranial magnetic stimulation (TMS). MEPs generated by focal electrical stimulation of the forelimb area of motor cortex were simultaneously recorded from the brain stem, contra- and ipsilateral forelimb and contralateral hindlimb muscles in rats subjected to transient MCA occlusion. The effect of ischemia on cortical activity and axonal conduction was differentially studied by proximal or distal occlusion of the MCA. Regional cerebral blood flow changes in the forelimb area were monitored by laser-Doppler flowmetry during ischemia and reperfusion. In addition, synaptic transmission within the forelimb area of motor cortex was examined by intracellular and extracellular recording of potentials generated by stimulation of the premotor area. No MEP response was recorded during ischemia. Upon reperfusion: (i) motor axons readily regained their excitability and cortical stimulation caused successive pyramidal volleys (recorded as D waves from the brain stem) and a MEP from contralateral paralytic muscles although synaptic activation of motor pathways was not feasible; (ii) the amplitude of pyramidal volley was increased; (iii) MEPs with a longer latency were recorded from the ipsilateral forelimb. In conclusion, differential recovery of synapses and axons after ischemia may account for some previously unexplained findings (such as preserved MEPs in paralysed muscles) observed in cortical stimulation studies of stroke patients.

Environmental influence on brain-derived neurotrophic factor messenger RNA expression after middle cerebral artery occlusion in spontaneously hypertensive rats

Enriched environment significantly enhances postischemic functional outcome. We have tested the hypothesis that housing in enriched environment stimulates gene expression for brain-derived neurotrophic factor. After ligation of the middle cerebral artery in male spontaneously hypertensive rats, they were housed in individual cages for 30h, then housed either in standard cages or in an enriched environment. The rats were killed two to 30days after the ischemic event. Cryostat coronal sections through the dorsal hippocampus (Bregma -3.3) were processed for in situ hybridization using a rat-brain-derived neurotrophic factor messenger RNA antisense oligonucleotide probe. Postischemic gene expression was significantly higher in standard rats than in enriched rats in contralateral and peri-infarct cortex and in most parts of the hippocampus two, three and 12days after the ischemic event, with a trend for higher-than-baseline levels in standard rats and lower-than-baseline levels in enriched rats. At 20 and 30days the values for both groups were below baseline levels. Contrary to our hypothesis, gene expression in rats postoperatively housed in enriched environment was significantly lower than in standard rats at a time when other studies have reported hyperexcitability in the ipsilateral and contralateral cortex. Should the low messenger RNA levels correspond to low protein synthesis, this might indicate that dampening of the early postischemic hyperexcitability may be beneficial. Low levels in both groups at 20 and 30days may correspond to loss of callosal connections in the opposite hemisphere and to horizontal cortical connections in the lesioned hemisphere.

Permanent reduction of seizure threshold in post-ischemic CA3 pyramidal neurons

The effects of ischemia were examined on CA3 pyramidal neurons recorded in hippocampal slices 2-4 mo after a global forebrain insult. With intracellular recordings, CA3 post-ischemic neurons had a more depolarized resting membrane potential but no change of the input resistance, spike threshold and amplitude, fast and slow afterhyperpolarization (AHP) or ADP, and firing properties in response to depolarizing pulses. With both field and whole-cell recordings, synaptic responses were similar in control and post-ischemic neurons. Although there were no spontaneous network-driven discharges, the post-ischemic synaptic network had a smaller threshold to generate evoked and spontaneous synchronized burst discharges. Thus lower concentrations of convulsive agents (kainate, high K(+)) triggered all-or-none network-driven synaptic events in post-ischemic neurons more readily than in control ones. Also, paired-pulse protocol generates, in post-ischemics but not controls, synchronized field burst discharges when interpulse intervals ranged from 60 to 100 ms. In conclusion, 2-4 mo after the insult, the post-ischemic CA3 pyramidal cells are permanently depolarized and have a reduced threshold to generate synchronized bursts. This may explain some neuropathological and behavioral consequences of ischemia as epileptic syndromes observed several months to several years after the ischemic insult.

Middle cerebral artery occlusion produces secondary, remote impairment in hippocampal plasticity of rats - involvement of N-methyl-D-aspartate receptors?

There is increasing evidence that focal cerebral ischaemia produces remote functional alterations that may substantially contribute to the post-stroke neurological outcome. Changes initially limited to peri-infarct areas may evolve and spread via transneuronal connections to other structures. In the present study we investigated whether focal ischaemia produced by 2-h occlusion of the middle cerebral artery (MCAo) in SD rats may influence the physiological function of the hippocampus. Three days later in vitro long-term potentiation (LTP) was studied in hippocampal slices from ipsi- and contralateral hemispheres. In rats with MCAo LTP was not-inducible in the ipsilateral hippocampus, while the contralateral side expressed stable potentiation (6.6+/-4.1 vs. 35. 0+/-8.0%, respectively). Treatment with 6-h i.v. infusion of an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MRZ 2/579 starting at reperfusion not only preserved but additionally enhanced ipsilateral LTP, while a slight insignificant decrease was observed in the contralateral side (77.0+/-18.4 vs. 20.8+/-6.5%). The study demonstrates post-stroke functional changes in the hippocampus that can be modulated by NMDA receptor antagonists.

Ionotropic glutamate and GABA receptors in human epileptic neocortical tissue: quantitative in vitro receptor autoradiography

Since a disturbed balance between excitatory and inhibitory amino acid receptors is suggested to be an important condition for epileptogenic cortical activity, the present study has focused on the analysis of the densities of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate, kainate and GABA subtype A receptors in neocortical tissue surgically removed from patients with focal epilepsy. The mean densities (collapsed over cortical layers I-VI) and the laminar distribution patterns of [3H]AMPA, [3H]MK-801, [3H]kainate and [3H]muscimol binding to AMPA, N-methyl-D-aspartate, kainate and GABAA receptors were determined with quantitative receptor autoradiography in the neocortex of patients with focal epilepsy and controls. The tissue probes used in the present study were functionally characterized by parallel electrophysiological investigations. From that, the different probes could be subdivided into a spontaneously spiking and a non-spontaneously spiking group. The mean density of [3H]AMPA binding sites was significantly increased (+37%) in the group of epileptic brains (n = 10) compared with controls (n = 10), but the mean densities of [3H]MK-801, [3H]kainate and [3H]muscimol binding sites were not significantly altered (-8%, +/-0% and -7%, respectively). The relation between the densities of all four binding sites were simultaneously displayed as polar plots in each single brain ("receptor fingerprints"). The consistent up-regulation of [3H]AMPA binding sites in all epileptic brains was found to be associated with a down-regulation of the N-methyl-D-aspartate receptor in four of the five non-spontaneously spiking cases, and an associated up-regulation of the N-methyl-D-aspartate receptor was seen in all spontaneously spiking cases. Finally, the laminar distribution of binding site densities was analysed, since the mean densities collapsed over all neocortical layers may obscure layer-specific alterations. Layer- and receptor- specific up- or down-regulations were found in epileptic tissue compared with controls. Moreover, the laminar distribution pattern of current sinks associated with epileptiform potentials in a spontaneously spiking cortical slice was found to be co-localized with local maxima of AMPA receptor densities. The present analysis of four ionotropic glutamate and GABA receptor subtypes demonstrates a consistent and significant up-regulation of [3H]AMPA binding sites in all cases of human focal epilepsy, which co-localizes with the occurrence of sinks in current-source-density analysis. The receptor fingerprint analysis suggests a subdivision of focal epilepsy into two subtypes on the basis of neurochemical/functional correlations: (i) a spontaneously spiking subtype with increased N-methyl-D-aspartate receptor density, and (ii) a non-spontaneously spiking subtype with decreased N-methyl-D-aspartate receptor density.

Periinfarct and remote excitability changes after transient middle cerebral artery occlusion

Transient middle cerebral artery (MCA) occlusion results in substantially smaller cortical infarcts than permanent MCA occlusion if reperfusion is initiated within the first few hours. Only little information is available on the long-term functional outcome of the cortical regions "salvaged" by early reperfusion. To address this issue we examined basic electrophysiologic parameters in vitro using standard extracellular recording techniques at 7 and 28 days after transient MCA occlusion (1- and 2-hour ischemia) in rats. Both neocortical areas ipsi- and contralateral to MCA occlusion were systematically mapped to delineate the extent of periinfarct and remote alterations. In the periinfarct region we found a significant reduction of field potential amplitudes up to 3 mm when measuring from the infarct border at 7 days and up to 7 mm at 28 days. Paired-pulse inhibition, an indicator of GABAergic transmission, was only moderately impaired in this region at 7 days and not significantly different from control at 28 days. Remote effects were observed both ipsi- and contralaterally. Ipsilaterally they were restricted to a region close to the midline (presumably motor cortex) and were most likely attributable to the degeneration of corticostriatal connections. The extent of the contralateral excitability changes was clearly related to the size of the neocortical infarcts with large infarcts resulting in the widespread reduction of field potential amplitudes and an impairment of paired-pulse inhibition. The results show that there is a relatively large periinfarct region with decreased overall excitability after transient MCA occlusion which is likely to have a profound effect on perilesional processes involved in functional recovery. Remote excitability changes may contribute to the functional deficit and are probably related to deafferentation.

Brain plasticity and stroke rehabilitation. The Willis lecture

Neuronal connections and cortical maps are continuously remodeled by our experience. Knowledge of the potential capabilityof the brain to compensate for lesions is a prerequisite for optimal stroke rehabilitation strategies. Experimental focal cortical lesions induce changes in adjacent cortex and in the contralateral hemisphere. Neuroimaging studies in stroke patients indicate altered poststroke activation patterns, which suggest some functional reorganization. To what extent functional imaging data correspond to outcome data needs to be evaluated. Reorganization may be the principle process responsible for recovery of function after stroke, but what are the limits, and to what extent can postischemic intervention facilitate such changes? Postoperative housing of animals in an enriched environment can significantly enhance functional outcome and can also interact with other interventions, including neocortical grafting. What role will neuronal progenitor cells play in future rehabilitation-stimulated in situ or as neural replacement? And what is the future for blocking neural growth inhibitory factors? Better knowledge of postischemic molecular and neurophysiological events, and close interaction between basic and applied research, will hopefully enable us to design rehabilitation strategies based on neurobiological principles in a not-too-distant future.

Recovery after damage to motor cortical areas

Until recently, the neural bases underlying recovery of function after damage to the cerebral cortex were largely unknown. Recent results from neuroanatomical and neurophysiological studies in animal models have demonstrated that after cortical damage, long-term and widespread structural and functional alterations take place in the spared cortical tissue. These presumably adaptive changes may play an important role in functional recovery.

Early training may exacerbate brain damage after focal brain ischemia in the rat

Early overuse of a lesioned forelimb, induced by immediate immobilization of the intact forelimb after a cortical lesion, has been reported to increase tissue damage and delay functional recovery. To investigate if early training without immobilization of the intact forelimb could increase tissue loss and reduce recovery, the middle cerebral artery was ligated distal to the striatal branches in 25 male spontaneously hypertensive rats. Control rats were housed in standard cages, training rats were transferred to larger cages allowing various activities and received additional special training 1 hour a day starting either 24 hours or 7 days after the ligation. The rats were tested on a rotating pole, in a leg placement test, and in a water maze and they were killed 6 weeks after the ligation. Delayed training resulted in the best overall performance; however, both training groups performed better than standard rats on the rotating pole. The cortical infarct volume was larger in the early training group than in the other two groups (P < .005), possibly related to increased glutamate release and peri-infarct cortical hyperexcitability.

Characterization of neuronal migration disorders in neocortical structures. II. Intracellular in vitro recordings

Neuronal migration disorders (NMD) are involved in a variety of different developmental disturbances and in therapy-resistant epilepsy. The cellular mechanisms underlying the pronounced hyperexcitability in dysplastic cortex are not well understood and demand further clinical and experimental analyses. We used a focal freeze-lesion model in cerebral cortex of newborn rats to study the functional consequences of NMD. Intracellular recordings from supragranular regular spiking cells in cortical slices from adult sham-operated rats revealed normal passive and active intrinsic membrane properties and normal stimulus-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively). Regular spiking neurons recorded in rat dysplastic cortex showed on average a significantly smaller action potential amplitude, a slower spike rise, and a less steep primary frequency-current relationship. Stimulus-elicited EPSPs in NMD-affected cortex consisted of multiphasic burst discharges, which coincided with extracellular field potentials and lasted 150-800 ms. These epileptiform responses could be recorded at membrane potentials between -50 and -110 mV and were blocked by -2-amino-5-phosphonovaleric acid (APV), indicating the involvement of N-methyl--aspartate (NMDA) receptors. Isolated NMDA-mediated and APV-sensitive EPSPs could be recorded at membrane potentials negative to -70 mV, suggesting that NMDA receptors are activated at relatively negative membrane potentials. In comparison with the controls, polysynaptic IPSPs mediated by the gamma-aminobutyric acid (GABA) type A and B receptor were either absent or reduced in peak conductance in microgyric cortex by 27% (P < 0.05) and 17%, respectively. However, monosynaptic IPSPs recorded in the presence of ionotropic glutamate receptor antagonists revealed a similar efficacy in NMD and control cortex, indicating that GABAergic neurons in microgyric cortex get a weaker excitatory input. Our data indicate that the expression of epileptiform activity in NMD-affected cortex rather results from an imbalance between excitatory and inhibitory synaptic transmission than from alterations in the intrinsic membrane properties. This imbalance is caused by an increase in NMDA-receptor-mediated excitation in pyramidal neurons and a concurrent decrease of glutamatergic input onto inhibitory interneurons.